The present invention relates generally to radio frequency (RF) signal transmissions through a dielectric barrier, such as a vehicle windshield, and more particularly relates to an improved, low-cost glass mount mobile antenna system.
The expansion of mobile and personal cellular telephones or telephone systems has been rapid and widespread during the last few years. Originally, cellular telephone systems were designed to provide communication services primarily to vehicles and thus replace mobile radio telecommunication systems. Advancements in technology and production have significantly decreased the cost of cellular service to the point at which cellular telephone service has now become affordable to a majority of the general population. Therefore, the "cellular telephone system" no longer strictly refers to cellular telephones, which originally were physically attached to and made a part of a vehicle. Cellular telephone service now includes portable, personal telephones which may be carried in a pocket or purse and which may be easily used inside or outside a vehicle or building.
One ultimate goal of the communications industry is to broaden the scope of cellular communication services by providing individuals with small, inexpensive, hand-held communicators by which the users may be reachable by voice or data communications with a single phone number, irrespective the location of the user. This proposed system has been generally referred to as a personal communication network/personal communication system ("PCN/PCS"). The PCN/PCS system is envisioned to be a wireless, "go anywhere" communication system which should for all intents and purposes eliminate the need for separate numbers for the office, home, pager, facsimile or car.
In anticipation of the development of PCN/PCS, many countries and communications providers have agreed upon international communication standards and have set aside a portion of the ultra-high frequency microwave radio spectrum as the bandwidth range which is to be exclusively dedicated for use by PCN/PCS. The entire bandwidth range expected to be used in PCN/PCS on a worldwide basis extends from about 1.5 GHz to about 2.4 GHz and individual countries have set aside different ranges, or bandwidths within this overall bandwidth for their national operations. For example, Japan has set aside from about 1.429 GHz to about 1.521 GHz, Europe has set aside from about 1.710 GHz to about 1.880 GHz and the United States has set aside from about 1.850 GHz to about 1.990 GHz. These different bandwidths all represent approximately 11%, or about 200 MHz of the total bandwidth set aside for PCN/PCS. The lower end of this overall bandwidth, 1.5 GHz is approximately two times higher than the standard frequency at which current cellular telephone systems operate at, namely 800 MHz.
The present invention is directed to low cost antennas for use in the ultra-high frequency operating ranges intended for PCN/PCS communications. Primarily, antennas for use in PCN/PCS communications will be mounted on vehicles to provide a mobile aspect to PCN/PCS. However, it is also envisioned that such antennas may be mounted on building window glass to further extend PCN/PCS services directly to a user's home or office. Both of these building and vehicle antennas will be of the glass mount type which will eliminate the need to drill holes through or otherwise modify a vehicle body or building wall.
Glass mount antennas typically utilize two modules which are mounted on the outside and inside surfaces of the window glass to transmit signals through the window glass between the opposing modules. The outside antenna module typically includes a vertically extending antenna radiating element, while the inside antenna module typically contains a connector or transmission feedline which may lead to a utilization device such as a telephone, pager, facsimile machine or the like. The outside and inside antenna module transmit RF signals between each other through the window glass. Loss occurs in glass mount antennas because they must transmit their signals through a dielectric material, such as the window glass and also must match the impedance of the outside antenna. Therefore, a window glass mount antenna typically has lower gain compared to a roof-mount, antenna which has a physical connection which extends through the vehicle body between inside and outside modules.
Previous glass vehicle cellular mobile telephone antennas have employed capacitive coupling in order to transmit RF signals through the glass of a vehicle window. In capacitively coupled antennas, two metal plates are positioned opposite each other on opposing surfaces of the window glass. These metal plates cooperate and act as a capacitor to transmit RF energy through the intervening window glass. Where the operating frequency of the communications system 800 Mhz, such as in a US cellular system, the metal plates are electrically small compared to the operating wavelength.
However, at the 1.5 GHz to 2.4 GHz frequency range of PCN/PCS, the RF signals begin to stray and the plates no longer act as capacitive couplers, but rather one of the metal plates acts as the primary radiating antenna element.
Capacitively coupled systems and associated impedance matching networks are generally described in U.S. Pat. No. 4,089,817, issued May, 1978 and U.S. Pat. No. 4,839,660, issued June, 1989. Capacitive coupling presents a number of disadvantages. As the operating frequency of the antenna enters the ultra-high frequency range, the electrically conductive plates must be increased in their size in comparison with the operating wavelengths to prevent any single one of the plates from becoming the primary radiating element in the antenna. Because the metal plates cannot be made large enough in comparison with the operating wavelength, a high impedance coupling of the nature of several hundred ohms occurs and cannot be avoided. This impedance will lead to high loss due to the leakage of the electrical field at high frequencies. In the ultra-high frequency bandwidth of PCN/PCS, even a small metal plate may no longer act as a capacitor element, considering the thickness of the vehicle glass and the stray capacitance. In such a situation, the circuit may bypass the signal and make it more difficult to match the high impedance of the antenna to the conventional 50 ohms of the utilization device, or telephone, used within the vehicle or building.
With the problems that occur at high operating frequencies, it is critical that an antenna system has a low pattern distortion. A conventional collinear array whip element does not have a uniform current distribution and the lower section of the whip typically exhibits the strongest radiation. When attached to a window of a vehicle, the lower section of the whip antenna element is blocked by the roof of the vehicle resulting in pattern distortion and deep nulls. At the 1.8 GHz of the PCN/PCS band, the situation becomes worse, because the length of the radiator element is typically only half that of those employed in the cellular bandwidth because of the doubling of the frequency.
A collinear array type whip antenna with a high feeding point may be provided by applying a decoupling sleeve or slot technology. This type of antenna typically has a 50 ohm to 75 ohm input impedance, which renders it difficult to adapt to capacitive coupling.
U.S. Pat. No. Re. 33,743 describes a capacitively coupled antenna system for coupling a coaxial feedline through a window glass, using a 1/4-wave antenna. However, at PCN/PCS frequencies, the 1/4-wave antenna suggested by this patent will have a length of approximately 1.7 inches which for all intents and purposes will be disposed beneath the roofline of a vehicle and which will result in severe pattern distortion and deep nulls.
Another approach is described in U.S. Pat. No. 4,939,484 issued Jul. 3, 1980, which discloses a coupling arrangement in which helical conductors are housed within outer conductors and are used to couple the RF signals through a window glass. This patent indicates that the size of the helical conductors and their housings must be fixed to satisfy the object frequency. In the 800 MHz operating frequency associated with conventional cellular communications systems, the helical cavity is designed for 200 MHz. However, at the ultra-high frequencies intended for PCN/PCS, and specifically at about 1.8 GHz, the helical conductor must be designed for 600 MHz. At this size, a significant drop of unloaded Q will occur because of the small helical conductor and the coupling coefficient attained by such an arrangement will not be enough to retain the 11% bandwidth preferred for PCN/PCS. Moreover, the helical conductor approach described in this patent is difficult to tune and is further difficult to manufacture because of its complex, three-dimensional structure.
The performance of the prior art antenna assemblies described above will degrade considerably for frequencies higher than 1.5 GHz. Prior art antennas are relatively low frequency systems as compared to the ultra high frequencies intended for PCN/PCS, and they utilize low Q, lumped LC elements, or semi-lumped elements provided by incorporating an LC circuit placed in a metal enclosure. The loss of such an LC circuit will increase considerably due to the low Q nature of such an antenna when used at higher PCN/PCS frequencies. PCN/PCS communication systems must operate at low power levels of about one watt and must provide a very wide range of coverage at the ultra high frequencies which comprise the bandwidth of such systems. The minimum bandwidth is near to 11%, and prior art antennas are simply not appropriate for operation in the PCN/PCS band because of their low frequency approaches.
U.S. Pat. No. 5,471,222, issued Nov. 28, 1995 and assigned to the assignee of the present application describes one antenna system which overcome the problems and disadvantages described above which occur in the PCN/PCS band. In that application, an antenna system is described wherein the inner and outer modules are provided with hollow metallic cavities which contain high Q ceramic resonators which couple the signal through the glass. The operation of such a system is very well suited for PCN/PCS applications. However, the structure disclosed therein is relatively costly.
Accordingly, a need exists for a glass mount antenna system which can operate effectively at the ultra-high frequencies intended for PCN/PCS of about 1.5 GHz to about 2.4 GHz with minimum losses and which is relatively inexpensive and easy to manufacture. Microstrip antennas, and particularly slot fed antennas, have been described in the literature and offer some promise over the prior art capacitive and inductive coupling systems. Microstrip antennas typically include a microstrip antenna, such as a patch or printed dipole, located on one substrate which is affixed to another substrate upon which a microstrip feedline is located. A ground plane is defined between the two substrates and typically contains an aperture therein through which the antenna patch and feedline are coupled. Such an arrangement is described by Pozar in "Electronics Letters", Volume 21, Number 2, dated Jan. 17, 1985.
The present invention is directed to an antenna apparatus utilizing microstrip technology and particularly, planar cavity slot coupling which is capable of desirable performance characteristics at ultra-high frequencies associated with PCN/PCS in which two coupling members are provided with planar cavities and exciter strips and are placed on opposite sides of a window glass to provide a through glass antenna assembly. The prior art simply fails to teach an appropriate structure to allow microstrip transmission of electrical signals through a dielectric medium such as window glass.
Thus, an object of the present invention is to provide an improved glass mount antenna system which has comparable overall performance in the PCN/PCS bandwidth to a ceramic resonator approach, but with much lower cost and with some advantages.
It is another object of the present invention to provide a glass mount antenna system adapted to operate at ultra-high frequencies which exhibits greater coupling efficiency and less pattern distortion which may be easily fabricated.
It is still another object of the present invention to provide a glass mount antenna assembly adapted for use at PCN/PCS operation frequencies for installation on either a vehicle or building window which utilizes aperture coupling on opposing coupling members.
It is yet a still further object of the present invention to provide a glass mount antenna assembly having opposing, aligned inside and outside modules, the inside module being connected to a utilization device, such as a telephone, the outside module being connected to a radiating element, the utilization device and radiating elements being respectively electrically connected to inner and outer coupling members formed from printed circuit boards, each of the coupling members having a planar cavity defined on their innermost opposing surfaces, the cavities being generally aligned with each other on opposite surfaces of the window glass, the inner and outer coupling members further having, on their outermost surfaces, an exciter strip which crosses the slots.
It is still another object of the present invention to provide a glass mount antenna assembly which utilizes aperture coupling to transmit RF signals through a window, the antenna assembly including inside and outside antenna modules, each antenna module including distinctive coupling plates which oppose each other, the coupling plates each having a ground plane formed on a surface thereof with a coupling aperture defined therein, the coupling plates further having an exciter strip on opposite surfaces of the ground planes, the exciter strips being disposed thereon generally perpendicularly aligned to the coupling plate slots, the two ground planes being aligned in the resonant direction to minimize loss.
It is still yet another object of the present invention to provide an inexpensive antenna apparatus having outside and inside modules adapted for mounting on opposite surfaces of a window, each of the inside and outside modules having a coupling plate which includes a printed circuit board, the outside module coupling plate including a metallic coating on one surface thereof which forms a ground plane, and the coupling plate further including on its opposite surface, an exciter strip having an elongated stub portion which crosses the cavity and which further includes an extension portion thereof to form a T-bar style exciter strip, the inside module coupling plate also including a metallic ground plane with a planar cavity and a T-bar, or cross, exciter strip on the coupling plate opposing surface.